SuttaCentral
Greetings dear SuttaCentral members. 
I’ve got a short question just like in the topic name: What is etymology of the term “Kruba Ajahn”, how it originated? and also: what exactly does the term mean?
As far as I can deduce from usage of the term, it means something like great master of thai forest tradition. But what exactly tells who is/was a “Kruba Ajahn” and who is just “Ajahn”?
Is there some sort of list of Kruba Ajahns?
With Metta 
and Anjal
A while ago I heard a talk from one of the Abhayagiri abbots (either Aj. Karunadhammo or Ñaniko) talking about this ethnology. He mentioned that there were two likely sources. One relating kruba to the word kru which is like guru and a more tenuous link to a Thai word for deva.
Not knowing Thai this is all I have and it’s a bit vague.
Kruu ครู in Thai comes from the Sanskrit word ‘guru คุรุ’, meaning ‘heavy; one with respectable conduct; one who teaches’. The word is used in Thailand to refer to teachers from kindy to high school though in the Royal Thai Navy, it’s our tradition to call teachers at the Naval Academy as well as senior supervisors as ‘kruu’.
Ajahn in Thai comes from the Pali/Sanskrit word ‘Acarya’, meaning “one who teaches”. In Thailand, it refers to high education institute teachers and those who have specialised knowledge.
In Thailand, Phra (=monk) Ajahn is used to call a monk who is senior and who teaches. Phra Kruu is an ecclesiastical title of a lower rank in the Thai monastic order.
Kruu Baa is a northern dialect traditionally to refer to a senior monk such as an abbot who is over 50 years old (generally 30 years in robe) and who teaches young lay men (Kula Butra).
Kruu Baa Ajahn is a phrase to refer to teachers as a generic term. For instance, we have to respect ‘Father, Mother and Kruu Baa Ajahn’. (A characteristic of the Thai language is to create a longer phrase with internal assonance to make it more poetic or to soften the meaning of the word.)
***Please note that the Thai spelling of the word Ajahn is อาจารย์ and it looks similar to Acariya with the last syllable bearing a silence marker. The word Ajahn here is written by western monks to reflect how the word sounds with no regard to the original spelling.
In the north of Thailand the popular etymologies for khruubaa are:
Garupi ācariyopi hotīti garupāti.
He’s a guru and an ācariya, therefore a khruubaa.
Garupādāne hotīti garupāti.
He’s in the lineage of gurus, therefore a khruubaa.
Though in Pali texts there doesn’t seem to be any such word as garupā.
Thank you all for your informative and helpful replies.
Thank you Dheerayupa for such detailed explanation. I wish I could give two “Solutions” but I can give only one. But thank you very much Bhante Dhammanando as well, because your reply was also very helpful. And thank you Bhante Akaliko for informing Dheerayupa about the question. And thank you Ayya Pasanna for interesting response and initiating comments.
~30 years in robe - beautiful. Deep respect to all Kruba Ajahns and I hope there will be more and more of them in this world for the benefit of all. 
With Metta.