This is a play on words, using the Japanese reading of the Latin letter O, which is オー (ō), for 0; for the digit 7, using first syllable of its kun'yomi; for the digits 2 and 1, using first syllables of their respective on'yomi. Finally, the word reads オナニー (onanī, “masturbation”).
Noun edit 0721オナニー • (onanī)
(slang) masturbation