During an extremely pleasant walk in the South Downs this weekend, we came across a copse. After hearing a description of what a copse is (a small thicket), I realised that I knew another word for it: coppice. It turns out that they are closely related, and that copse comes from the phonetic reduction of coppice.
This is what the OED has to say about it:

OF. copeïz, couppeiz, colpeïz:–late L. type *colpaét€écium ‘having the quality of being cut’, f. colpaét– ppl. stem of colpaére, to cut with a blow, f. late L. colpus (Salic Law), earlier colapus (Alemannic Law) blow, stroke:–L. colaphus, a. Gr. j¾kauo| blow, cuff. (The AFr. and ME. form was latinized in later times as copecia, copicia.) As in other Fr. words ending in an s sound, the plural was orig. the same as the sing. copys; this led to the Eng. sing. being frequently made copy, coppy, which is now very common in the dialects. On the other hand, the vowel of the final syllable was, as in the -es, -is, -ys of plurals, often dropped, leaving cops, surviving in the form copse.