is a combination of なら 'if', まだ 'still', and the adverbial particle
しも, which has the primary purpose of strengthening the word that comes before it. When used, this structure will often be translated as '(A) is fine, but not (B)', or '(A) is fine, but it's different when it's (B)'.
is a phrase that usually separates the first and second half of sentences, so will be attached to the end of the (A) phrase, marking the statement that is acceptable.
If it was hot, that would be fine, but because it is also humid, I don't want to go.
If it was 2 or three days, we could wait, but 2 weeks? We can't wait that long.
If it was only once, okay, but don't you feel any sort of way being told the same thing everyday?
Due to the way that しも alters まだ, a more literal translation of this grammar point can be thought of as 'if it's (A), then of course there's still that, but as for (B)'.
しも is a classical particle that is not frequently used in modern times outside of set phrases or expressions like ならまだしも
, 今（いま）しも 'right this moment', and だれしも 'absolutely anybody'. In all cases, it merely strengthens the word directly before it.
I was just about to leave the house.
Everyone has an attachment to a fishing rod they have used for many years.