lethargic_man | חתן תורה

Ooh, this is fascinating: apparently originally the term was not חתן תורה (Ḥāthān Torāh), the bridegroom of the Law, but חתם תורה (Ḥāthām Torāh), the sealer of the Law.

This ties in the mutation of ־ם to ־ן in Mishnaic Hebrew, and later scribes switching it back except in plural endings: if people hadn't realised this was such a mutation, it would result in the above change of meaning.

(Also, a note from elsewhere: I read that some of the earliest שמחת תורה piyyutim are clearly meant to accompany a spring festival; and that these would have been used in the three and a half year cycle when the Torah reading was completed in the spring.