In Arabic literature, 'Khatama' is used to indicate 'completion', 'conclusion', 'finality' or to put it simply - termination of an affair/event/object/verb. This is the meaning in which renowned Arabic scholars/litterateurs have used this word.

The views of famous Arabic litterateurs on this word are quoted below:

A. In his famous work 'Qamoos-e-Arabi', among the best and reliable lexicons of the Arabic language, Adrodalin says:

1. Khatama - 'He sealed, sign or stamp, to design, to make a sign. For example Khatama Shahadatan - He sealed the witness.
   
   Or as the Quran says -

   خَتَمَ اللّهُ عَلَى قُلُوبِهمْ وَعَلَى سَمْعِهِمْ وَعَلَى أَبْصَارِهِمْ غِشَاوَةٌ وَلَهُمْ عَذَابٌ عظِيمٌ

   'Allah has set a seal upon their hearts…' (Surah Baqarah, verse 7).
   
   

2. Khaatama. For instance, 'Khaatama Shai'in' means 'the end of thing' or Khaatama Quran - 'He completed the Quran'
   
   
3. Khataama - A seal made up of sticky mud which is impressed upon a writing or a thing.
   
   
4. Khaatim - Khaatam - The last one in a gathering/assembly of people E.g. Khaatimun Nabi'een - Last one of the Prophets or Khateemul Anbiya - Last one of the Prophets i.e. Prophet Muhammad (s.a.w.).
   
   (Please not that the above examples have been advanced by the author of Qamoos and not by me.)
   
   
5. Khatam also refers to the ring which is employed for sealing and for signing.
   
   
6. Makhtum - Sealed one or signed one
   (Qamoos-e-Arabi - English, page 702 - 703)

B. Likewise, another leading authority in Arabic literature - Allamah Ibne Manzur in his magnum opus - Lisanul Arab unravels the meaning of Khatama.

1. Khatama - Seal
   
   
2. Makhtum - Sealed or locked

From the viewpoint of Arabic literature - Khatama and Tab'a share the same meaning i.e. sealed and there is no room for anything new to enter it.

For e.g.

مَّا كَانَ مُحَمَّدٌ أَبَا أَحَدٍ مِّن رِّجَالِكُمْ وَلَكِن رَّسُولَ اللَّهِ
وَخَاتَمَ النَّبِيِّينَ وَكَانَ اللَّهُ بِكُلِّ شَيْءٍ عَلِيمًا

'Muhammad is not the father of any of your men, but he is the Apostle of Allah and the Last of the prophets.' (Surah Ahzab, verse 40)

Allamah Ibne Manzur sheds light on the evolution of the word Khaatam into 'ring', a meaning that finds greater acceptance among the Bahais and Qadianis. In the past, the seal (khaatam) was employed to emboss an authority's acknowledgement of official documents. Since this 'embossing' was done through an imprint of the ring, which was often found on the finger of the official/authority, the embossing came to mean 'a ring' or 'an adornment'.

C. Raghib Isfahani, another noted litterateur, in his celebrated work - 'Al-Mufradaat' outlines the meaning of this word.

1. Khaatam - For e.g. Khaatamal Quran intehaayatan ilaa akherehi. 'Completing a reading of the Quran till its end.'
2. Khaatamun Nabiyyin - Last of the Prophets.

D. Leading Arabic litterateur, Fakhruddin Turaihi in 'Majmaul Bahrain' explains the meaning of Khaatamun Nabiyyin in Surah Ahzab, verse 40:

'Muhammad (s.a.w.) is the last prophet and there is no prophet after him.'

For e.g. - Man khaatamun lahu beqiyaamil lail summa maata falahul jannat
'One who stands in worship till the end of the night and then passes away, then Paradise is for him.'

The references I have cited above have been compiled by litterateurs across different centuries. So the meaning of 'Khatama' hasn't really evolved to come to mean 'an adornment'. These references have been drawn from a mix of Ahle Sunnah and Shia sources, so there is unanimity across the primary Islamic sects about the meaning of 'Khatme Nabuwwat', which is that Muhammad (s.a.w.) is the last Prophet to be raised by Allah.

To adopt an interpretation of a word different from its oft-used meaning begs a deep insight in the language, an intelligent rationale and an endorsement from the Sunnah. Lets take the first premise - when Bahais talk of a different interpretation for 'Khatama' from the literal one, are they saying it is based on a study or research into Arabic literature that suggests that adopting the metaphorical meaning of Khatama (i.e. adornment) is a preferable option to taking the more common and obvious literal interpretation of seal, final, last? If they are, then I eagerly await to see a piece of research that rivals the works of Ibne Manzur and Raghib.

As a matter of fact 'Khatam' means final in Persian, Urdu and even Hindi. These languages have undoubtedly borrowed the word from Arabic, but chose the more popular and widely prevalent literal meaning over its vague and uncommon metaphorical counterpart. This is expected, because when words transcend across languages, then it is the most commonly employed meaning that gains currency and not its metaphorical implication. From the perspective of our discussion, it only serves to reinforce that Khatama's most popular interpretations are seal, final, last and not adornment.

In terms of a rationale, there is no reason to suggest that Allah meant it in a meaning different than seal, final, last. Even if for argument's sake one adopts the metaphorical meaning of adornment, it does not close the door on adopting the literal meaning of seal, final, last.

In terms of an endorsement from the Sunnah, this metaphorical interpretation is conspicuous by its absence, unless one goes about interpreting the scores of traditions that assert Khatamiyyat in a manner to support one's conclusion, in this case, adornment. If anything, the Quran and Sunnah have eulogised Prophet Muhammad (s.a.w.) as the best of Prophets, Islam as the best of nations and Muslims as the best of people. And if Allah has saved the best for the last, then you do have the last Prophet and the 'adornment' of Prophets in the persona of Prophet Muhammad (pbuh). In my view, the Muslims have reason to thank the Bahais and Qadianis for giving 'Khatamun Nabiyyin' a totally new dimension!