na-adjective, noun which may take the genitive case particle 'no', noun
Describes an action or event that happens without warning, often with a nuance of surprise or lack of preparation. Commonly used as an adverb with に.
出し抜けに雨が降り出した。
Suddenly it started raining.
彼は出し抜けに辞表を出した。
He abruptly handed in his resignation.
Compound of 出し (dashi, 'putting out') + 抜け (nuke, 'omission' or 'slipping through'). The exact derivation is uncertain, but it likely comes from the idea of something slipping out or appearing without prior notice.