expression, i-adjective
trivial; no big deal; not amounting to much
Used to downplay something as unimportant or not worth worrying about. Often appears in casual conversation.
See also: 大した
この問題は大したことないよ。
This problem is no big deal.
彼の
What he says doesn't amount to much.
大した is a prenominal adjective meaning 'great' or 'considerable', often used in positive contexts, while 大したことない is a negative phrase meaning 'not a big deal'.
Phrase composed of 大した (considerable, great) + こと (thing) + ない (not), literally 'not a considerable thing'. The exact historical derivation is uncertain, but it is a common colloquial expression.