Words for servants, ploughmen and related people in Celtic languages.

Proto-Celtic	*ambaxtos = servant
Gaulish	*ambaxtos = vassal, high-ranking servant
Old Irish (Goídelc)	amus = servant amsach = mercenary
Irish (Gaeilge)	amhas = hireling, servant, mercenary, hooligan amhsach = wild, unruly amhasóireacht = hooliganism
Scottish Gaelic (Gàidhlig)	amhas [au.əs] = savage, wild person, madman amhsach = wild, uncontrollable, stupid, dull
Proto-Brythonic	*ammaɨθ [amˈmaɨ̯θ] = servant, worker, labourer
Middle Welsh (Kymraec)	amaeth = ploughman, husbandman, farmer, agriculture
Welsh (Cymraeg)	amaeth [ˈameɨ̯θ / ˈamei̯θ] = ploughman, husbandman, farmer, agriculture, ploughmanship, tillage amaethadwy = farmable, cultivable amaetha(f), amaethu = to farm, husband, plough, cultivate amaethdir = arable land, land suitable for cultivation, farm land amaethdy = farmhouse amaethddyn = agriculturalist, farmer amaethedig = farmed, cultivated, cultured amaethyddiaeth = agriculture, farming
Cornish (Kernewek)	ammeth = agriculture, farming
Old Breton	ambaith = agriculture, farming

Etymology: from the Proto-Celtic *ambi- (around),‎ *ageti (to drive) and‎ *-os, from the Proto-Indo-European word *h₂m̥bʰi-h₂eǵ- (drive around) [source].

The English word amassador comes from the same root, via the Middle English ambassadore from the Anglo-Norman ambassadeur (ambassador), from the Old Italian ambassadore, from the Old Occitan ambaisador (ambassador), from ambaissa (service, mission, errand), from the Medieval Latin ambasiator (ambassador), from the Gothic 𐌰𐌽𐌳𐌱𐌰𐌷𐍄𐌹 (andbahti – service, function), from the Proto-Germanic *ambahtaz (servant), from the Gaulish *ambaxtos [source]. The word embassy comes from the same Gaulish word [source].

Proto-Celtic	*wastos = servant
Gaulish	*wassos = young man, squire
Old Irish (Goídelc)	foss = attendant, man-servant, servant
Proto-Brythonic	*gwass = boy, servant
Middle Welsh (Kymraec)	guas, gwas = boy, lad, servant
Welsh (Cymraeg)	gwas [ɡwaːs] = boy, lad, stripling, youngster, young man; servant, attendant, employee, officer, vassal, slave gwasanaeth = service, attendance, a ministering, office, duty, employment gwasanaethu = to serve, be a servant, attend, wait upon, minister
Middle Cornish (Cernewec)	guas = servant gwas = a youth, servant, one of the common people, a mean person, a fellow, rogue, rascal gwasanaeth = attendance, service, bondage, slavery
Cornish (Kernewek)	gwas = chap, fellow, guy, servant gwas hwel = workman gwas ti = housemaker
Old Breton	guos = vassal, man, husband, farmer
Middle Breton	goas = vassal, man, husband, farmer (who rents a farm)
Breton (Brezhoneg)	gwaz [ˈɡwaːs] = (young) man, vassal, valet, servant, husband, mermaid

Etymology: possibly comes from the Proto-Indo-European word *upo-sth₂-o-s (standing beneath) [source].

The English word vassal comes from the same Celtic roots, via the Old French vassal, the Medieval Latin vassallus (manservant, domestic, retainer), from the Latin vassus (servant) from the Gaulish *wassos [source].

Old Irish (Goídelc)	seirbísech = auxiliary, ancillary, servant, agent
Irish (Gaeilge)	seirbhíseach = servant
Scottish Gaelic (Gàidhlig)	seirbheiseach [ʃerʲevɪʃəx] = servant, servitor seirbheisiche = servant
Manx (Gaelg)	shirveishagh = attendant, clergyman, minister, servant, server, vassal

Etymology: from the Old French servise (service, servitude, vasselage), from the Latin servitium (slavery, servitude, service), from servus (servant, serf, slave) [source]

Words marked with a * are reconstructions.

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Gerlyvyr Cernewec, Dictionaire Favereau, TermOfis, Le dictionnaire diachronique du breton, Geriafurch, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic