Finn (n.)

"low land covered wholly or partly by water, a marsh abounding in coarse vegetation," Old English fenn "mud, mire, dirt; fen, marsh, moor," from Proto-Germanic *fanja- "swamp, marsh" (source also of Old Saxon feni, Old Frisian fenne, Middle Dutch venne, Dutch veen, Old High German fenna, German Fenn "marsh," Old Norse fen, Gothic fani "mud"), from PIE *poino-, from root *pen- "swamp" (source also of Gaulish anam "water," Sanskrit pankah "bog, marsh, mud," Old Prussian pannean "swampland"). Italian and Spanish fango, Old French fanc, French fange "mud" are loan-words from Germanic. The native Latin word was limus or lutum.

"of or pertaining to Finland or its inhabitants," c. 1790, from Finn + -ish. Earlier was Finnic (1660s as a noun, in reference to the language). Related: Finno-.