Where does “ceithern” come from?

ceithern (Middle Irish) comes from Old Irish ceithern, from Latin quaterniō, from Latin quaternī, from Latin quaternus, from Latin quater, from Latin quattŭor, from Portuguese quarto, from Old Portuguese quarto — and.

ceithern (Middle Irish): A band or troop of soldiers or fighting men, kern

Definitions

A band or troop of soldiers or fighting men, kern

Ancestry of “ceithern”, step by step

Step	Language	Word	Meaning
1	Old Irish	ceithern	band of soldiers
2	Latin	quaterniō	The number four (e.g. on a dice)
3	Latin	quaternī	—
4	Latin	quaternus	four each; four at a time
5	Latin	quater	four times
6	Latin	quattŭor	four; 4
7	Portuguese	quarto	ordinal form of quatro; fourth; quarter; fourth;...
8	Old Portuguese	quarto	quarter
9	Latin	quārtus	fourth
10	Proto-Indo-European	kʷeturtos	—
11	Proto-Indo-European	kʷetwóres	four
12	Proto-Indo-European	kʷe-	and

Words derived from “ceithern”

Every word from Proto-Indo-European kʷe- →Every word from Proto-Indo-European kʷetwóres →Every word from Proto-Indo-European kʷeturtos →