The name Rainer is a boy's name of German origin meaning "wise army". Rainer and is often added to lists like Boy Baby Names That Mean Wise and discussed in our forums with posts like "Boys Makeover- Two Middles".
From the experts:
Rainer is one of those baby names that is completely obscure -- only 19 boys received the name in the U.S. in 2011 -- yet easy to understand and an intriguing possibility. Part nature name and part tradtional Teutonic choice, Rainer can claim a place among the two-syllable r-ending baby names so fashionable for boys today.