Someone who helps another in need for compassionate motives and with no thought of reward.
This expression derives from the Biblical parable, Luke 10:30/33 (Miles Coverdale's Version, 1535):
10:30 Then answered Iesus, and sayde: A certayne man wente downe from Ierusalem vnto Iericho, and fell amonge murthurers, which stryped him out of his clothes, and wounded him, and wente their waye, and left him half deed.
10:31 And by chauce there came downe a prest the same waye: and whan he sawe him, he passed by.
10:32 And likewyse a Leuite, wha he came nye vnto the same place and sawe him, he passed by.
10:33 But a Samaritane was goynge his iourney, and came that waye, and whan he sawe him, he had compassion vpon him,
The figurative use of the term began in the 17th century. In 1649, Peter Chamberlen published a book titled The Poore Mans Advocate, or, Englands Samaritan.