It doesn't actually mean anything as such. It was the nearest word the young Berry Gordy could think of when he founded his first record label in 1958, and wanted to call it 'Tammy Records' after the Debbie Reynolds movie. Realising that he may run into copyright problems, Gordy adapted his label name, calling it Tamla instead. A year later, he formed his second label, 'Motown', a version of the phrase 'mo'town', short for 'motor town', the nickname for Detroit, where it all started - remember Martha Reeves imploring everyone to 'Don't forget the motor city!' in her hit 'Dancing In The Streets'?
Not long after that, Berry Gordy amalgamated his two labels into one - you know the rest!