Orcus was the name for Pluto, the god of the infernal regions, hence we can easily understand the explanation of hel-deofol. Orc, in Anglo-Saxon, like thyrs, means a spectre, or goblin.
"This story is of long ago. At that time the languages and letters were quite different from ours of today. English is used to represent the languages. But two points may be noted. (1) In English the only correct plural of dwarf is dwarfs, and the adjective is dwarfish. In this story dwarves and dwarvish are used, but only when speaking of the ancient people to whom Thorin Oakenshield and his companions belonged. (2) Orc is not an English word. It occurs in one or two places but is usually translated goblin (or hobgoblin for the larger kinds). Orc is the hobbits' form of the name given at that time to these creatures, and it is not connected at all with our orc, ork, applied to sea-animals of dolphin-kind.
"Orc" is an Old English word, very obscure in the 1930s; Tolkien probably ran across its use while translating Beowulf into contemporary English, or else in other old documents, and adapted it into his legendarium. Prior to this fantasy does not generally use the term; E. R. Eddison in The Worm Ouroboros for example uses Goblin and Gobinland, but not "Orc." William Blake uses "Orc" as "Albion's angel" in America A Prophecy (1793), but this is a different variation on the concept of "orc" as a term for evil spirit. When Tolkien is talking about "orc" in terms of dolphins, etc., he means the contemporary orcus, and also the older use of the term orc to refer to whales - as in Orlando Furioso (1516).
The orc as we know it today essentially derives from Tolkien's conception of orc and goblin, which is what was adapted by Gary Gygax and Dave Arneson for Dungeons & Dragons (1974) as essentially malevolent humanoids.
Tolkien's specific conception of orcs was also influenced by other factors, I might point to: