Green’s Dictionary of Slang

1. a fool.

1863		‘Mark Twain’ in Smith Twain of the Enterprise (1957) 97: This is an appalling statement, but it is a true one. I guessed it from remarks made by that disreputable old cottonhead, Bill Stewart, who as good as promised me ten feet in the ‘Justis,’ and then backed down again when the stock went up to $80 a foot.
1877		Eaton Democrat (OH) 29 Nov. 4/6: ‘I’m very much obliged to you both, I’m sure,’ said Cottonhead, with a radiant smile [...] ‘W-h-a-t,?’ shrieked Mrs A., [...] ‘You forlorn idiot’.
1889		Arizona Wkly Enterprise (Florence, AZ) 19 Jan. 1/2: Cottonhead — Aw say, old chappie, haw lawng cawn a fellow live withawt brains?
1905		Mexico Missouri Message (MO) 2 Feb.) 1/3: [of a mule] The prices are given for the better class of mule colts [...] and it is fair to suppose that [...] the ‘little cottonhead’ was a drag on the market.
1969		in DARE.
2001		The Dungbeatles ‘He’s Got a Big Truck’ 🎵 And when I see a cottonhead a doing fifty-five / In the left lane a’ Sunday driving just barely alive / He’ll turn red a’ screamin’ and will flash his middle digit / What comes around goes around to this mental midget.

2. a state of forgetfulness.

2002

Remifemin 🌐 Well, some people call it cottonhead, some call it ‘menofog’ but most people just think of it as menopause memory.