Originally, 'soro' (in old times, Samorahu'サモラフ,' Saburahu'サブラフ,' and so on) was a verb meaning to serve in the vicinity of a person of high rank ('samurai' also originated from this).
「候」（古くはサモラフ、サブラフなど）は元来、貴人の傍に仕える意の動詞であった（「さむらい」もこれに由来）。