Where does “Lachlan” come from?

Lachlan comes from Scottish Gaelic Lochlann, from Old Irish loch meaning lake, ultimately from Proto-Indo-European lókus with the same sense.

Ancestry of “Lachlan”, step by step

Step	Language	Word	Meaning
1	Scottish Gaelic	lachlann	—
2	Middle Irish	Lochlann	—

Lachlan — Etymology, Origin & Word History | Etymology Explorer