noun
straggling hair; loose strands of hair
Refers to hair that has come loose from a tied or styled arrangement, often around the nape or temples.
ポニーテールにしたら、後れ毛がたくさん出てしまった。
When I put my hair in a ponytail, a lot of loose strands came out.
彼女はうなじの後れ毛をかき上げた。
She tucked the straggling hair at the nape of her neck behind her ear.
アホ毛 refers specifically to a single unruly strand sticking up, often on the top of the head, while 後れ毛 is about multiple loose strands, typically at the back or sides.
Compound of 後れ (okure, 'lagging behind') + 毛 (ke, 'hair'), referring to hair that has fallen behind or escaped from a style.