Synonyms for Underside:

n.

defect, root, shortcoming, flaw, imperfection, Achilles' Heel, inadequacy, failing, fault, weakness, deficiency. bottom (noun)
depths, foundation, sole, foot, underneath, root, bottom, base, nadir, basement.
lower side (noun)
bottom, undersurface, base.
underside (noun)
undersurface, bottom.

Other synonyms:

n.
bottom.

Usage examples for underside

  1. The wood has deteriorated considerably over the years and the joints are loose, but, within the limits of the existing structure, the plow beam can easily be set in such a position that its sloping rear end lines up with the slope of the underside of the standard. – John Deere's Steel Plow by Edward C. Kendall
  2. The support of my box, a strip of wood running from the underside of that little feeding table to the house wall, had loosened its lower nail, and one day, when some passing touch of grippe kept me in bed, with Sigurd sitting upright on a chair beside me, playing nurse, a plump jay lit heavily upon the edge of the shelf and screeched with fright as it shook and slid beneath him. – Sigurd Our Golden Collie and Other Comrades of the Road by Katharine Lee Bates