Lughaidh wrote:
Of course it's an Irish word (what else would it be? see the spelling

), .
Isn't it true, though, Lughaidh, that it might well have been a Scottish Gaelic word (Scottish Gaelic words having similar spelling but not all of which are used in both languages) ?
Dinneen (1927) has:
Quote:
Cliaṁain ... a relation by marriage, esp. a son-in-law ; al. a father- or mother-in-law (Con.) ...
Cliaṁain, a., related by marriage
Cliaṁnaċt, a., relation[ship] by marriage ...
Other materials specific to the Connemara dialect confirm the broader meaning of "a relation by marriage".
So, although FGB states, "son-in-law", it is not limited to "a son-in-law" (but if not specified, that might be it's most likely interpretation, at least outside Connacht).
It appears to be related to
cleamhnas "(arranged) marriage".