build up

verb5

1. •
   To erect; to construct.

   • The motor bogies are built up from plates and rolled sections, and have cast-steel, one-piece axleguards, with manganese liners. — 1956
2. •
   To accumulate: to increase incrementally or continually.

   • Ever since the secretary left, the letters in my inbox have started to build up.
3. •
   To strengthen.

   • They had to build up their fortress to protect against attack.
4. •
   To close up by building.
5. •
   In solitaire card games, to place a card over another card of lower value. (e.g., place 5♦ over 4♣)