The penis of a pre-op transgender woman. Described as feminine because:
1. It's on a woman.
2. Feminizing hormones often change the anatomy and functionality of the penis. (eg. it doesn't get as hard, it doesn't really ejaculate, and it has a different mouthfeel)

The term was further popularized by the YouTuber ContraPoints who discusses the term in the video Tiffany Tumbles. This definition is almost taken almost verbatim from her video.

an extremely small appendage usually attached to the groin area of a male. often smells like a mixture of papaya and mango. can be used for mild stimulation, if any. when stimulated itself it secretes a liquid which is comparable to snuggle fabric softener. owner will usually cry after secretion and try to cuddle.

A penis, that is a complete opposite to how a normal masculine penis should look like.

Instead of a big, juicy, fat, veiny cock, a feminine penis are usually small, soft, usually pinkish, and are delicate (holy fuck this is so gay)

1. the male genitalia seen as looking feminine in its proportions, colour, carriage etc..
2. the penis of a 'modestly endowed', pre-operative male-to-female transsexual.
3. the arbitrary parameters dictating whether the male sex organ is thus (i.e., 'feminine').

Etymology:
The male genitalia has historically often been used as a primitive, superficial measure of masculinity among the strata of 'lower-educated' males of many societies. As such, a smaller or less 'masculine-looking' penis can sometimes be labelled "feminine" in appearance (i.e., femininity being considered the default weaker / lesser gender). In this sense, the description can be both an insult (e.g., for a "straight" and / or insecure male) and compliment (e.g., for a MTF transsexual girl).