no; nope
Emphatic, casual denial. Stronger than いいえ; often used when rejecting an offer or correcting a misunderstanding.
「もう一杯どう？」「いいや、結構です。」
"How about another drink?" "No, I'm fine."
「これ、君の？」「いいや、違うよ。」
"Is this yours?" "Nope, it's not."
いいえ is the polite, neutral way to say no, while いいや is a casual, more emphatic denial used with friends or in informal talk.
Derived from the interjection いや (iya, 'no') with an emphatic lengthening of the initial vowel. The exact historical development is uncertain, but it is a common casual variant.