-ier1

1.
variant of -er1 , usually in nouns designating trades:
collier; clothier; furrier; glazier.
Origin
Middle English -ier(e), variant of -yer(e) (cf. -yer), equivalent to -i- v. stem ending + -ere -er1, probably reinforced by Old French -ier < Latin -ārius -ary (cf. soldier)

-ier2

1.
a noun suffix occurring mainly in loanwords from French, often simply a spelling variant of -eer, with which it is etymologically identical (bombardier; brigadier; financier; grenadier); it is also found on an older and semantically more diverse group of loanwords that have stress on the initial syllable (barrier; courier; courtier; terrier). Recent loanwords from French may maintain the modern French pronunciation with loss of the final r sound (croupier; dossier; hotelier).
Origin
< French, Old French < Latin -ārius, -āria, -ārium -ary; cf. -aire, -eer, -er2
British Dictionary definitions for -ier

-eer

suffix
1.
(forming nouns) indicating a person who is concerned with or who does something specified: auctioneer, engineer, profiteer, mutineer
2.
(forming verbs) to be concerned with something specified: electioneer
Word Origin
from Old French -ier, from Latin -arius-ary

-ier

suffix
1.
a variant of -eer brigadier
Word Origin
from Old English -ere-er1 or (in some words) from Old French -ier, from Latin -ārius-ary
Word Origin and History for -ier

word-forming element indicating occupation, from French and Old French -ier, from Latin -arius (also see -er (1)). Nativized and used to form English words (glazier, hosier, etc.; also see -yer).