(1) My mother-in-law is so frigid, when she opens her mouth a light goes on.

(2) I wouldn't say that my mother-in-law is ugly, but whenever she puts on her make-up, the lipstick backs down the tube.

How funny is (1)?

What does the bold part in (2) mean?

Thank you very much, all the time!
1) not very. It would be better with the word 'cold' instead of 'frigid'. You can know if your mother-in-law is a cold person (unemotional, unfriendly) but it is a bit weird to complain about her attitude towards sex! As a 'my wife' joke it would be ok, but not about your mother-in-law.

2) It tries to get away from her. It retreats back down the lipstick tube.
Emotion: embarrassed Wow, I didn't notice it was sexual!

Where is it sexual? The "light goes on" part?
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The word 'frigid', apart from meaning 'cold', could also mean 'averse to sexual intercourse'.
Ah, I got it. It's the "mouth opening" part that is sexual. It implies B-job.
Nope. 'The light going on' just refers to the similar mechanism in the refrigerator.

Just curious but the word 'teleostomi' was originally proposed by R. Owen to include bony fishes with a terminal mouth.
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No, like I said, if you replace 'frigid' with 'cold' it's fine and means exactly what you meant. It has no sexual element apart from the word 'frigid'. B told you what it means but if you didn't understand that definition:

Definition 1 in my dictionary: Frigid. Sexually unresponsive: unable or unwilling to respond sexually, to enjoy sexual intercourse, or to have orgasm during intercourse.

It would make a great joke (better than the original with substitution of cold) if you made it about your wife rather than your mother-in-law. English humour is built on puns and double meanings, so the combination of 'cold temperature meaning of frigid/comparing her to a fridge' and 'sexually frigid/cold wife/frigid also means cold and sounds a bit like fridge' makes the joke a different one, but a lot funnier. But can you see why it's not appropriate for your mother-in-law joke now?

So if you want to make it a slightly rude joke, keep frigid but make it about 'my wife'.

If you don't want to make it a slightly rude joke, change it to cold and keep it about your mother-in-law.