στις κακές / στις καλές
Why are these expressions feminine plural? Is there an implicit word (un mot sousentendu)?
There should have been such a word, for sure, but I have never wondered what, or have I heard someone stating it.
It could be either one among
στιγμές, ώρες, μέρες or other similar nouns.
There is also the expression
Εἰναι στις μαύρες του meaning he's depressed (same 'omission').
Also:
Έχει τις κλειστές του (=
He's not in a mood to go out).
Similarly:
Δεν είναι στα καλά του (=
He's out of his mind), where the 'omitted' noun is in neutral plural, and the relevant
Έλα στα συγκαλά σου (=
get yourself together - a somewhat dated phrase).
The expressions
(Πήγαινε) στο καλό! (= Farewell / Fare well / Fare thee well) or
Άντε [or Άει] στο καλό! (with the same meaning as the previous one or, ironically, =Get out of here!) may be of the same kind.
We never cared for the 'missing' nouns, I think, which must have been long forgotten.