boostáer [b?st?r] (plural boostáers) noun 1. vigorous promoter of something or somebody: an enthusiastic promoter or supporter of something or somebody, e.g. of a team 2. somebody or something that causes improvement: somebody or something that improves, strengthens, or encourages somebody or something (usually used in combination) a morale-booster 3. shoplifter: somebody who engages in shoplifting (slang)