An etymological dictionary of French

Yet another interesting discovery from the 19th - early 20th century: this dictionary (warning: big pdf file), about 500 pages in length.

An example from achever:

ACHEVER, va., to finish. The Lat. caput, towards the end of the Empire, and in Merov. times, took the sense of an end, whence the phrase ad caput venire, in the sense of to come to an end: 'Filium filabo de quo Justinus Imperator, nec Augusta, ad caput veniro non possint', says Narses in the Ancient Chronology quoted by Gregory of Tours. We also, in like manner, find ad caput venire for 'to finish', in Fredegaire's Epit. ch. 65 (Monod). Venire ad caput naturally produced the French phrase venir à chef = venir à bout. Aucun d'eux ne put venir à chef de son dessein...whence the sense of chef = an end, term, conclusion. Sur cette route, au chief de chaque journée, il y a de beaux palais...From this chief, O. Fr. form of chef, in sense of term, end, comes the Fr. compd. achever = venir à chef, to end, finish.

Most entries are not quite this detailed.