Linctus is a thick liquid medicine for a cough. You take a small amount by mouth, usually from a spoon. The word is mainly British, and it sounds like a chemist’s (a pharmacy’s) word, the kind you might still read on a bottle. In many other places people just say cough syrup, and younger speakers often do the same even in Britain. Though actually, the name comes from a Latin word for licking something up: the old picture is medicine you lick or sip, not a pill you swallow dry. It is not a word you need for everyday chat.