The wrong side of the blanket
Euphemism for being born out of wedlock.
This expression, which denotes illegitimacy, has never been clearly explained and we aren't sure when and where it originated.
The first known use of it in print is in Tobias Smollett's comic novel Humphry Clinker, 1771:
Thof my father wan't a gentleman, my mother was an honest woman - I didn't come on the wrong side of the blanket, girl - My parents were marred according to the right of holy mother crutch, in the face of men and angles - Mark that, Mary Jones.
[Note: the spelling mistakes are deliberate, for humorous effect.]
With 'the wrong side of the tracks' there is a clear reference to being raised at the poor end of town. Getting out of the 'wrong side of the bed' is an allusion to starting the day on the wrong foot, but as to the derivation of the 'the wrong side of the blanket', there is no pat answer. Presumably there was also a right side of the blanket, but there aren't any known references to that either. Chalk this one up as an etymological mystery.