The story
From dos (“plural of do (“something that can or should be done”)”) + and + don’ts (“plural of don’t (“something that must or should not be done”)”).
Etymology from Wiktionary, CC BY-SA 4.0.
dos and don'ts
Today it means: A set of things that one should do or not do in a particular situation; a set of guidelines or rules that one has to or ought to follow. Full definition →