Noun

1. A glue or paste made from starch.
2. Low cloud, fog or smog.
   • 1993: Harry Furniss, Memoirs - One: The Flying Game

     The sky was thick with dirty gray clag

   • 2001: Colin Castle, Lucky Alex: The Career of Group Captain A.M. Jardine Afc, CD, Seaman and Airman

     This programme included practice interceptions, simulator training, day flying, night flying, clag flying -- in addition to... [a footnote states that clag flying was Air Force slang for foul weather flying.]

   • 2004: David A Barr, One Lucky Canuck: An Autobiography

     We went along in the clag for what seemed like an eternity [a footnote defines clag as low cloud cover]

3. Unburned carbon (smoke) from a diesel locomotive or multiple unit.

Verb

1. To encumber
   • c1620:Thomas Heywood, Thomas Heywood's Art of Love: The First Complete English Translation of Ovid's Ars Amatoria

     As when the orchard boughes are clag'd with fruite

   • 1725: Edward Taylor, Preparatory Meditations

     Can such draw to me/My stund affections all with Cinders clag'd

2. To stick, like boots in mud
   • 1999: "A queen of a Santee kitchen, pre-war", quoted by Mary Alston Read Simms in the Introduction to Rice Planter and Sportsman: The Recollections of J. Motte Alston, 1821-1909

     Wash the rice well in two waters, if you don't wash 'em, 'e will clag [clag means get sticky] and put 'em in a pot of well-salted boiling water.