The story
Earlier as ifs and 'ans, plurals of if and 'an (or an, en, or and), dialectal for then, influenced by the conjunction an (meaning if) (See OED entry on and conj.2). Thus it meant ifs and thens. Brewer's analyzes it as ifs and ans, with the meaning ifs and ifs, and that the spelling ands is a mistake. In any case, it was later supplemented with buts.
Etymology from Wiktionary, CC BY-SA 4.0.