Fully-sheathed is for when a penis is fully in its sheath from respective wiki pages: erection: When an image/animation shows a character with a visibly stiff penis or an underwear tent caused by an erect penis.

Erections are produced by increased blood flow into the penis caused by arousal, such as seeing an attractive enough woman's big breasts or man's big pecs upon undressing, engaging into sex, and other things that cause arousal. Erections make it easy for the penis to penetrate a character's pussy, anus, or other penetrable orifices.

If the penis is obviously erect but not directly visible, tenting or penis_outline may also apply. Bulge can also apply as well.

For full erections only. If the penis is only partially erect, use half-erect instead.

half-erect: When a character's penis is not fully erect but not quite flaccid either.

flaccid: Used when a penis is visible but not erect. Often it is pointing down, hanging loosely, or just sitting on top of the balls not doing anything in particular.

Not to be used if the penis is hidden entirely within the body. Consider using fully sheathed or genital slit instead.