"highest point," Old English top "summit, crest, tuft," from Proto-Germanic *toppa- (source also of Old Norse toppr "tuft of hair," Old Frisian top "tuft," Old Dutch topp, Dutch top, Old High German zopf "end, tip, tuft of hair," German Zopf "tuft of hair"); no certain connections outside Germanic except a few Romanic words probably borrowed from Germanic.
Few Indo-European languages have a word so generic, which can be used of the upper part or surface of just about anything. More typical is German, which has Spitze for sharp peaks (mountains), oberfläche for the upper surface of flat things (such as a table). Meaning "highest position" is from 1620s; meaning "best part" is from 1660s. To go over the top is World War I slang for "start an attack," in reference to the top of the trenches; as "beyond reasonable limits, too far" it is recorded from 1968. Top of the world as "position of greatest eminence" is from 1670s. Top-of-the-line (adj.) is by 1950.
"toy that spins on a point," late Old English top, probably a special use of top (n.1), but the modern word is perhaps via Old French topet, which is from or influenced by a Germanic source akin to the root of English top (n.1). As a type of seashell, first recorded 1680s.
"put a top on," 1580s, perhaps mid-15c., from top (n.1). Earlier "cut the top off, shave the head" (c. 1300). The meaning "be higher or greater than" also is first recorded 1580s. Meaning "strike (a ball) towards its top" is from 1881. Related: Topped; topping. To top off "to finish" is colloquial from 1836; in sense "fill up, add more to to bring to fullness" it is from 1917.
"being at the top," 1590s, from top (n.1). Top dollar "high price" is from 1942. Top-drawer (1920) is from British expression out of the top drawer "upper-class." Top ten in popular music is from 1945 ("Billboard"). The top dog is the one uppermost in a fight, from 1868 in figurative use, opposed to the underdog.
But if the under dog in the social fight runs away with a bone in violation of superior force, the top dog runs after him bellowing, "Thou shalt not steal," and all the other top dogs unite in bellowing, "This is divine law and not dog law;" the verdict of the top dog so far as law, religion, and other forms of brute force are concerned settles the question. [Van Buren Denslow, "Modern Thinkers: What They Think and Why," 1880]