expression, noun which may take the genitive case particle 'no'
(not) even a little; (not) a tiny bit
Used in negative sentences to emphasize that there is absolutely none of something, not even a hair's breadth. Often with ない or similar negative forms.
彼の話には毛ほども真実はない。
There isn't even a shred of truth in his story.
その計画に毛ほどの興味もない。
I don't have the slightest interest in that plan.
少しも is a more common and neutral way to say 'not even a little', while 毛ほど is more emphatic and vivid, literally comparing to a hair's breadth.
まったく means 'completely (not)' and is a stronger, more absolute negation, while 毛ほど emphasizes the lack of even a tiny amount.
Compound of 毛 (ke, 'hair') and ほど (hodo, 'extent, degree'), literally 'even the amount of a hair'. Used idiomatically in negative contexts to mean 'not even a tiny bit'.