Sanskrit साधु sādhu is "holy man, saint" and eariler "effective, correct, good", from sādh "to be accomplished or fulfilled or effected or settled, be successful, succeed".
The Proto-Indo-European root is *sē[i]dh-, sīdh-, sədh- "to strive for a goal". This became Greek ἰθύς ithus "straight". This was borrowed in ithyphallic, describing the straight phallus carried on Bacchic festivals. It's also used for the poetic metre used in Bacchic hymns.