1. To lay stones under the sills of a building, on which it is to rest; to support by some solid foundation.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• undering,
• underging,
• underpant,
• undertain,
• underoing,
• underming,
• underpend,
• undergir,
• uderpin,
• unerpin,
• undrpin,
• unnderpin,
• underpinn,
• undesoil,
• undarwn,
• underunk,
• unsleepin,
• enderin,
• undefiy,
• Underdic

• underage,
• under wraps,
• undersea,
• underdone,
• underhung,
• underrating,
• underline,
• undertaker,
• underwriter,
• underspend,
• underlie,
• under arms,
• undermanned,
• underpass,
• undertaking,
• underprice,
• undergird,
• undertow,
• underslung,
• undermine,
• underhand,
• undercover,
• underlying,
• underwater,
• underpay,
• under it,
• undoer,
• underarm,
• underfed,
• underpart,
• underskirt,
• undertone,
• undersoil,
• underpants,
• undersign,
• underdog,
• underworld,
• unadorned,
• underfur,
• underspent,
• underact,
• underneath,
• underway,
• underweight,
• underbid,
• understand,
• underplay,
• underling,
• underlay,
• underwrite,
• undergo,
• under,
• under fire,
• underhanded,
• underexpose,
• underdown,
• underground,
• undershirt,
• untier,
• undershrub,
• underwear,
• underwing,
• undercut.

The adverb form of "underpin" is "underpinningly".

The adjective form of the word "underpin" is "underpinning".

The plural form of the word "underpin" is "underpins"

The singular form of the word "underpin" is "underpin".