no, i can't either, i already checked my dictionary, and at first i thought it was a mistake, but so many sites say it's an Old German name meaning 'land', that i assume it must be right.
But...i've checked the site of the Meertens Instituut about Dutch firstnames and i've looked at Lammert, which is a variant of Lambert and i found that this name consists of land- and -bert , meaning 'shining (of the) land'. Well, this makes sense now that i come to think of it. I recently looked up a surname for someone (the name Sambad). And i found out then that Samberg is a variant of Sandberg and that Sambach is a variant of Sandbach. Apparently, the -nd- placed before a b can change in -m- . Thus, Lamar was originally Landar or Landmar. This would mean that the meaning of the name is incomplete and should be 'famous land', because the rootform -mar (as in Dietmar) means 'famous'. Or maybe Landar is just a variant of Land and the suffix -ar is maybe meaningless. Does this make more sense?