Hobby horse

[Rihanna]

(hä-bē-ˌhȯrs)
n.
I.

A toy horse, usually comprised of a figure of a horses head attached to a stick.

A toy horse that, via various mechanics, stimulate the riding of a horse through rocking.

(Etymology I- Derived possibly from the Obby Oss, an English folk tradition wherein Morris Dancers dress as horses, which, in itself, is probably derived from the IRISH HOBBY, a breed of horse, now extinct, that the Obby Oss was probably made to resemble)
II.

A favourite topic.

(Possibly from Lawrence Sternes Tristram Shandy, who uses it, quite frequently, in the third sense)
III.

A female of loose virtue and even looser undergarments, a woman who is easy to get, a lady who uses sex as a means of earning legal tender. See Prostitute, loose woman, and strumpet.

(Derived from def. I, a toy horse, she can be ridden by all)

[Dave Paprocki]

A large, thick dick (resembles a Horse Cock) [Hobby Ho]

[Donald Trump]

To ride someones ass, to bully or intimidate with power; To bugger your subordinates

[Agnes]

This act includes, but is not limited to the following:
1.) (v) Hiding the salami.
2.) (v) Having your head in your boyfriends ass.
3.) (n) tucking your balls in your ass, or the ass of a stranger, or cellmate.

[Little_Girl]

when you stick panties in a girls ass (like a horse tail) and use another pair around her neck like reins (for choking) and then go at it doggy style.