'broad-topped' is a modern compound formed in English from the adjective 'broad' and the past-participial adjective-form 'topped' (from 'top' + '-ed'), literally meaning 'having a broad top'.

'broad' comes from Old English 'brād' meaning 'wide'; 'top' (and hence 'topped') comes from Old English 'topp' (or Middle English 'top(p)e'), and the compound adjective developed by combining them in Modern English.

Individually, 'broad' originally meant 'wide' and 'top' meant 'summit' or 'upper part'; combined as 'broad-topped' the phrase has the straightforward, literal meaning 'wide at the top', which has remained stable.