The sense I’m interested in is in OED hum n.4.
Etymology: Croatian, = hill.
A small, usually conical, hill characteristic of karst topography.
1921 Geogr. Rev. 11 602 (caption) General view of a polje strewn with hums.
OED gives no indication how or when the Croation word first appeared in English. Can anyone help? There are Hum- names for streams and small geographical areas in the north east of England, which made me wonder if these two senses of hum- are related or from different sources? The English names predate 1921, possibly by centuries.