os is an old preposition meaning 'above'. Prepositions used to take the dative case, and cionn is the dative of ceann (head) (note: some nouns still have datives in use). So, lit. Above my head
Is the dative taught anywhere in the Duolingo course? I keep seeing it crop up, but only when I'm getting something wrong. ;-)
No its not. It's only used in imthe caighdeáin for 5 words and fossilized phrases
Okay, thanks. I'll just memorize once I get them wrong often enough to sink in.
would it then be permissible to say os mo cheann ? or is the chionn here because os chionn is an idiom?
I don't know, but you should probably be worried about what's behind you.
So you know how os means above as mentioned os cionn, then what does 'comhair' mean as in os comhair? Or is that not related?
The meaning of comhair in os comhair is “presence” — thus, os comhair = “upon presence” = “in front of” or “opposite”.
Ok, so is this literal? Or can it be interpreted as "what did i miss?" Or both?
It wouldn't be understood as "what did I miss?". But it also shouldn't be taken too literally - here are just a few examples from the NEID of os cionn meaning "over" or "above", rather that "over head":
"books were stacked on the table" - bhí leabhair carntha os cionn a chéile ar an mbord
"superimposed layers" - sraitheanna os cionn a chéile
"fines in excess of €1000" - fíneálacha os cionn €1000
"to marry above your station" - pósadh os cionn do chéimíochta
"half the population is above sixty" - tá leath an daonra os cionn seasca
"dawn broke over the hills" - d'éirigh an ghrian os cionn na gcnoc
I suggested "overhead" keeping the concept of "ceann" as head, but it was rejected!