>>> then it's a phrase of some kind (the name of the kind is known to linguists, and was known to me for a while)
>>>
>>>"Idiom" would do, though I suspect you're looking for something less open and shut. ;-)
>>
>>It was something more specific, but no matter how much I remember and try, the words won't come to me.
>
>colloquialism?

Irreversible binomials. I had to search there™ (in a forum from home where the™ trademark sign is used liberally).