Europeans are more down-to-earth here: Germans call it fett, fat; the French say gras, also fat. The Italians have neretto (black) or grassetto, and Spaniards go for letra negra or just negrilla. The Oxford English Dictionary gives c.1871 as its earliest quotation for bold in the sense of boldface type. (The use of boldfaced as describing an impudent person goes back to 1692, not really all that much earlier.) What did we call boldface type before that? No trace.

The fact is that prior to the nineteenth century we didn’t call bold type anything because it simply didn’t exist. Paul Luna in Anniversary Essays on Johnson’s Dictionary edited by Jack Lynch and Anne McDermott (CUP, 2005) informs us “The use of bold type for headwords in English dictionaries seems not to have come about until the 1870s, some thirty years after the introduction of the first boldface types, called Clarendon, by a London type-foundry [Robert Thorne’s Fann Street Foundry]. In the sixteenth and early seventeenth centuries, black-letter had provided a color contrast analogous to the use of boldface with roman.” Originally bold types were cast in larger sizes since they were really intended for advertising and posters.

But why did we English speakers opt for a word meaning courageous rather than the more obvious fat, heavy or black?  Well, recall that as time passes the meaning of simple words can migrate. In earlier times bold also meant big, plump, well-filled. One example quoted by the OED from 1787 is “Being a bolder and better grain, weighed heavier”. Thorne’s original faces do seem to have been referred to as fat at the time. Maybe Victorian prudery took over and demanded a less physical term.

Nowadays we have semi-bold and bold, extra bold, heavy, grotesque, and yes, fat. There’s no reason why you can’t call that typeface you just designed whatever you want, and there are no hard and fast boundaries between these terms, but I list them roughly in ascending order of fatness. Fonts in use has a nice survey of fat faces, with lots of graphic examples.