Lacking energy is a common feeling that can be described in many ways. INERT (5 letters) means lacking in energy or motion, LIMP (4 letters) means weak and without strength, SLUGGISH (8 letters) means slow and inactive, LETHARGIC (9 letters) means having little or no energy, LOGY (4 letters) means dull and heavy, SLACK (5 letters) means not tight or taut, ANEMIC (6 letters) means lacking vigor or energy, sapless (7 letters) means lacking vitality or energy, and LISTLESS (8 letters) means lacking energy or enthusiasm.