What it means
っこない expresses the speaker's strong conviction that something is impossible or will not happen. It attaches to a verb's masu-stem and is often used to reject an expectation or insist that a person could never do something. The expression is emphatic and somewhat colloquial, so it is common in speech and informal writing rather than in highly formal situations.
Usage note
っこない is much stronger and more subjective than a neutral negative expression such as ない or ません. The polite counterpart っこないです is possible, but the expression itself still retains a conversational and emphatic tone.
How to use it
彼はその約束を忘れっこないです。
He will never forget that promise.
彼はその約束を忘れっこない。
He will never forget that promise.
Example sentences
あの慎重な先生が、準備不足のまま試験を始めっこない。
There is no way that such a cautious teacher would start the exam without adequate preparation.
小学生の妹が、あんな重い家具を一人で運びっこない。
My elementary-school-aged younger sister has no chance of carrying such heavy furniture by herself.
忙しい姉が、約束の