Middle English betreuthe: from be- (expressing transitivity) + truth. The change in the second syllable was due to association with troth.

• plight from Old English:

  In the traditional marriage ceremony the bride and groom each say ‘I plight thee my troth’, meaning ‘I pledge my word’. Plight means ‘to promise solemnly’, and pledge (Middle English) is probably a distant relative. Troth is an old variant of truth, meaning ‘giving your word’ and still preserved in betroth (Middle English). The other meaning of plight, ‘a predicament’, is from Old French plit ‘fold’, suggesting the idea of a difficult or complicated situation. Other words from plit include Middle English pliant (Late Middle English) literally ‘foldable’; and pliable (Late Middle English); pliers (mid 16th century) tools for bending things; and ply (Late Middle English) in the sense of ‘thickness’ as in plywood (early 20th century). (The other ply as in ply with drink, is simply a shortening of apply, see appliance). Pleat and plait (Middle English) are further relatives. Compliant (mid 17th century) looks as if it should be a relative, but its immediate source, to comply (early 17th century), originally came from Latin complere ‘to fulfil, accomplish’, although compliant later developed senses influenced by its similarity to pliant.