tick [ME] The tick shown as a ✓ first meant ‘to pat, touch’ and goes back to medieval English, where it was related to tickle [ME], although its history is obscure. This is also the tick used to imitate the sound of a clock, and in ticker, or the heart, a sense first used in the USA at the end of the 19th century. The ‘bloodsucking parasite’ sort of tick [OE] is a different, older word which gives us the expressions tight as a tick or as full as a tick [L17th] for ‘very drunk’, both of which refer to the way ticks swell as they gorge themselves on blood. Both forms of the phrase have the additional meaning ‘be full after eating’, but the more recent tight as a tick plays on two senses of *tight, which can mean both ‘drunk’ and ‘stretched taut’. When you buy on credit or on tick, you are using yet another word, which is an abbreviation of *ticket. The ticket in question is an IOU promising to pay the money due, but there is also the suggestion of a pun on the reputation of moneylenders as ‘bloodsucking parasites’. Both on tick and on the ticket date back to the 17th century.