Merriam-Webster Collegiate Dictionary - best

 I. adjective, superlative of good  Etymology: Middle English, from Old English betst; akin to Old English bot remedy — more at better  Date: before 12th century  1. excelling all others the ~ student  2. most productive of good ; offering or producing the greatest advantage, utility, or satisfaction what is the ~ thing to do  3. most, largest it rained for the ~ part of their vacation  II. adverb, superlative of well  Date: before 12th century  1. in the ~ way ; to greatest advantage some things are ~ left unsaid  2. most those ~ able will provide needed support  III. noun  (plural ~)  Date: before 12th century  1. the ~ state or part  2. one that is ~ the ~ falls short  3. the greatest degree of good or excellence  4.  a. one's maximum effort do your ~  b. a ~ performance or achievement ran a new personal ~  5. ~ clothes Sunday ~  IV. transitive verb  Date: 1863 to get the better of ; outdo  V. verbal auxiliary  Date: 1914 had ~ you ~ listen BEST  biographical name Charles Herbert 1899-1978 Canadian (American-born) physiologist