demacrar

Used almost exclusively in its reflexive form, demacrarse, meaning to become emaciated or gaunt, typically from illness, grief, or exhaustion. The adjective demacrado ("gaunt," "haggard") is much more common in everyday description than the verb itself.

• Se demacró mucho durante la enfermedad. — He became very gaunt during the illness.
• El actor se demacró para el papel. — The actor made himself look gaunt for the role.
• Su rostro se había demacrado con los años. — His face had grown haggard over the years.

Using the non-reflexive demacrar for a person's own appearance — you almost always need demacrarse (reflexive) when describing someone growing gaunt, not the bare transitive form.