Being tongue-tied means being speechless or having difficulty speaking, possibly due to being surprised, nervous or frightened. It also means to be afflicted with tongue-tie, or a shortness of the frenum; this binds the tongue to the floor of the mouth, causing limited or problematic tongue movement.
The medical use of tongue-tied is much less common than the colloquial use. The common usage was first used between 1520 and 1530, while the medical community didn't adopt the term until approximately 20 years later. The phrase comes from the image of a tongue being tied in knots, causing difficulty speaking and communicating.