The Penguin Guide to Plain English

immunity / impunityThere is a slight overlap of meaning between these two words. The w ord im m unity is used medically of the bodys ability to resist disease and more generally of freedom from legal obligations and from official restraints (In return for agreeing to assist the authorities, the former criminal was granted immunity from prosecution) . Impunity is exemption from charge or penalty and thus now describes a state of safety ( He ridiculed the authorities with im punity).

imply I inferTo imply something is to convey it by what is said, perhaps indirectly. If a father informs his unemployed son that there is work to be had in a nearby firm, he is probably implying that the son should go and apply for a job there. In fact the word imply is rarely misused. What goes wrong generally is that the word infer is used as though it meant the same as imply. By every thing she said, she inferred that I was lazy should be: she implied that I was lazy. In modern usage to infer is not to convey a message but to deduce one (From his sickly appearance and lack of appetite I inferred that he was ill).

inapt I ineptA thing which is apt is highly suitable to its purpose. Thus what is inapt is inappropriate. The word is also used to mean unskilful, that is, lacking in aptitude. Inept is originally the same word spelt differently but we tend to apply it to a degree of inappropriateness and lack o f skill that implies clumsiness or stupidity ( It was inapt of him to miss his m others funeral, and inept to imagine that no one would notice).

informant I informerThe history of these two words is curious. In present-day usage, an inform ant is simply a person who conveys information, while an inform er is someone who lays information against another, bringing to light offences against the law. In the eighteenth century, however, the word informant was used in this latter sense, and the word inform er could be used to mean one who animates and inspires. Thus in Pope we find Nature! informer o f the human heart.