Fòram na Gàidhlig

Am Faclair Beag has smodaig, glan, and sgoinneil for "cool" and "groovy".

The notion of "cool", "neat", "sweet", "wicked", "sick", etc., etc., to express something that is fashionable, socially acceptable, innovative, special, or agreeable, is ubiquitous and used by all people of all ages. The term that seems to have stood the test of time is "cool", and is used in expressions like "I'm cool with that", "Be cool about it", and especially "that's cool", or just "cool!"

Some languages just borrow "cool". What do modern Gaelic speakers use?
("groovy" is not cool today)

I've never heard "smodaig", but I've heard "glan" and "glan fhèin".

Well, based on the map data I'd say smodaig is a Uist phrase, so maybe that's why you haven't heard it?

If there's any words that makes the cut it's sgoinneil, gasda and glan, but there's no word that covers everything "cool" does. Usually the same sentiment is addressed using "nach" or "fhèin" with an adjective. These are just off the top of my head and the English for them is super loose you could probably use any for any situation. I haven't put laghach grinn brèagha anabarrach but you'll see them too. Personally I still say cool in English if it calls for it. Irish has cula-bula (spelling >_<). These are mostly just emphatic ways of saying something was good.

tha sinn rèidh -- we're cool
tha sinn air còrdadh -- we're cool (made up)
glan fhèin -- cool! nice one
fada air ais -- uncool
sgionneil -- sweet, cool
math dha-rìribh -- yas
tha sin gasda -- neat/cool (an idea/a watch or something)
dìreach (fhèin) sgionneil -- awfa cool
bi dòigheil -- be cool (don't not be dòigheil)
nach math sin -- int that cool/something
gabh do shocair -- be cool (chill)
shin agad thu -- cool (cheers,nice one)
nach tu a tha a' coimhead spaideil -- looking good, cool outfit
abair thusa gun robh sin fìor mhath -- man that was really cool

snasmhor