society > authority > subjection > service > at one's service or ready to serve [phrase] > as phrase of politeness (2)
phr. Yours (etc.) to command (†to be commanded): i.e. for you to command or dispose of.
at your service: used as a polite deferential formula in greeting or valediction, (formerly) at the end of a letter, etc. Cf. servant, n. phrases…