Why is it “I better not (+verb)” instead of “I better don’t (+verb)”?

This question will seem weird to a native speaker because “I better don’t” sounds inherently wrong and unusual. But if you think about it, it’s an irregularity; normally when a verb is negated and there is no auxiliary already present, the auxiliary “do” is inserted and usually contracted with the “not” to form “don’t”. The exceptional construct “I better not” baffles some non-native speakers.

What is the etymological history behind the construction “I better not (+verb)”? Did this sentence use to contain an auxiliary which has disappeared?

The full phrase is I had better or I'd better, and it has a rather obscure and interesting history. It seems to be derived from an Old English phrase like betera me*, literally better to me and having the meaning it would be better for me.

The object me changed to the subject I, and had showed up in the phrase some time in the Middle English period. This probably arose by association with I would better, which is why it's always had and not a present or other form of to have. As other answers have mentioned, where in Old English it took the infinitive, the phrase now takes the subjunctive, hence I'd better go, not I'd better to go.

Note that best may be used as well: I (had) best avoid rattlesnakes.

*I don't know how to inflect this.

It's because the full form is "I had better not [do something]" - but the "had" is sometimes dropped in colloquial language.

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