Lacking in number (of a military unit, sport team, etc.)(adjective)
Lacking in strength or durability (of a material or structure)(adjective)
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Use "understrength" in a sentence
"Of course, before the war Tolbukhin would not have called the understrength regiment he led a front."
"With Rio Ferdinand and Steven Gerrard ruled out for England, both countries are likely to field understrength and inexperienced lineups, so it's safer to stick to the outright result."
"That understrength line-up here did at least have Ledley King in its midst."