Groove
This word has evolved from its original meaning in jazz slang as a sexual euphemism.
1. The vagina. The female groove.

2. The ass, both male and female groove.

3. Musical term: All parts of a band working together. May have been a euphemism for lovers pleasuring each other simultaneously, or taken from the ruts in an old dirt road making a ride smoother.

4. Hippie term: Taken from music, but relating it to life, social situations, and nature. Generally feeling good, in tune and in the rhythm of nature. Enjoying life.

Original sexual meanings are seldom used or understood anymore.
"Groovin" meant rubbing the genitals in one of the grooves. Used as a simple form of birth control, somewhat similar to withdrawel.(See also frotting or fronting)

"Groove Bone" meant a penis. To "make it to the groove" meant something similar to "third base". "Shake that groove thang" meant to shake the vagina or ass either while dancing or having sex.

Less sexually oriented meanings are still sometimes used.
"Hip to the groove" means aware of what gives pleasure; often used as a music term.

To be "In the groove" means to be in a pleasurable place, either musically, socially, or in life or nature. Generally, running smoothly.
"Groovin' together" enjoying each other's company. Appreciating each other. Generally enjoying the situation and the soroundings.
(Lyrics) "Shake that groove thang"
(Lyrics) "Groovin' on a Sunday afternoon. Doin' all the things you like to do."
(Lyrics) "Just because a records got a groove doesn't mean it's in the groove".