Why is 69 a dirty number?

Answer:

The number 69 (sixty nine or sixty-nine) is not a dirty number. It is a number just like any other number.

There are, however, some sexual connotations with the number 69. In pop culture, 69 refers to reciprocal oral sex.

Because the numbers 6 and 9 are basically inversions of one another and their shape is similar to that of a human body (in profile view), the imagery associated is that of a couple performing oral sex on each other at the same time.

The Os in the numbers signify the heads of two people while the ascending and descending stems are that of the rest of the two human bodies. When put together, this is what appears in one's imagination.

The number 69 has become somewhat of a verb in recent years. It is not too uncommon for a person to refer to the usage as of "69'ing (sixty-nining) with someone."