/ Dictionary / Index L L sill: Meaning and Definition of Find definitions for: L sill Pronunciation: [key] — Carpentry. Carpentry. a sill for a building frame composed of a plate resting on the basement wall and a header or joist at the outer edge of the plate. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. LSI LSM Related Content Daily Word Quiz: lunar Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language