verb (used with object), un·der·lapped, un·der·lap·ping.
Examples from the Web for underlap
The placket of a skirt should have an underlap extending well below the opening.
In many cases, it is advisable to have an underlap of the material.
On the left side of the skirt sew an underlap of sufficient length to extend well below the end of the opening.
This is the type most likely either to overlap or underlap, depending upon the prison rgime and the after-parole circumstance.Criminal Types|V. M. Masten