Infirm comes from Latin infirmus ('weak, frail'), from in- ('not') + firmus ('strong, steadfast'). The prefix supplies the first semantic turn, while the base carries the lexical core of the word. As an adjective, it carries the sense "physically weak, especially through age or illness", with the older formation still helping explain the modern use. Relevant historical forms include Latin infirmus and Latin firmus. Derivative forms include infirmly, infirmity, infirmary; related words include firm, infirmary, affirm, confirm.