HAD, preterit tense and participle passive of have; contracted from Sax.haefd, that is, haved; as, I had; I have had In the phrase, 'I had better go, ' it is supposed that had is used for would; 'I'd better go.' The sense of the phrase is, 'it would be better for me to go.'
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI