diligence (n.)

mid-14c., "constant and earnest effort to accomplish what is undertaken," from Old French diligence "attention, care; haste, speed" and directly from Latin diligentia "attentiveness, carefulness," from diligentem (nominative diligens) "attentive, assiduous, careful," present-participle adjective from diligere "single out, value highly, esteem, prize, love; aspire to, be content with, appreciate," originally "to pick out, select," from dis- "apart" (see dis-) + legere "choose, gather," from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')."

Sense evolved through time from "love" through "attentiveness" to "carefulness" to "steady effort." Legal sense "attention and care due from a person in a given situation" is from 1620s. From the secondary French sense comes the old useage of diligence for "public stage coach" (1742; dilly for short), from a French shortening of carrosse de diligence.