Plainly
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penis
noun/ˈpiːnɪs/
meaning in plain English
The body part on males used for peeing and for sex.
Picture a pen is stuck to a guy—pen is + is = penis, the male part for peeing and sex.
for example
The female clitoris is like the male penis in some ways.
Imagine a little boy at the pool suddenly realizing he needs to pee—he knows exactly where to aim that handy little body part. Or picture a dog lifting its leg on a fire hydrant, instinctively using its penis the way nature intended, no instructions needed.