A tiny amount, a smidge, just a little bit. Usually shows up as a softener in classic British understatement, like a tad late or a tad expensive when it’s actually fully gone to bits. Sounds polite and harmless, which is exactly why it’s funny when the situation is clearly a mess.
"The bill’s a tad more than I expected. Oh yeah, how much? Four hundred quid. Mate, that’s not a tad, that’s a full-blown crisis, get the kettle on"