The Moin dictionary, one of the most trusted dictionaries of Farsi, has written "Sapõn" (if I'm doing the accent right) as a non-Arabic form of the word صابون /saabun/, and this can be traced to Latin sāpō, soap. According to Wiktionary:
From Proto-Germanic saipǭ, from Proto-Indo-European seyb-, *seyp- (“to pour out, trickle, strain”). Cognate with Old English sāpe (“soap, salve”), Old English sāp (“amber, resin, pomade, unguent”), Latin sēbum (“tallow, grease”). More at soap.
Because the verb -osha is for washing other things (usually the dishes).
The verb -fua is for washing clothes. Then there is -nawa for washing parts of the body.
These verbs are conjugated in the standard way, including "ninafua (nguo)" = I wash (clothes) and "anafua (nguo)" = he/she washes (clothes).
You can look up "wash" here: