[1] The lexicographer draws a connection between ko/b-alos, used in Aristophanes, Knights 450, and the verb ko/p-tw "cut". The Etymologicum Gudianum s.v. ko/pis derives it from ko/pis "liar", which it explains oxymoronically as "brief, sharp in words, or babbling". (ps.-Zonaras uses the same gloss, deriving it directly from ko/yai.) More plausibly, the Etymologicum Magnum and Etymologicum Gudianum s.v. ko/balos derive it from kopi/s via kopi/balos "?knife-thrower".
In fact, the verb kobaleu/w (kappa 1895) appears to preserve the original meaning of the stem, "porter"; so also the 3rd c. CE noun kobalismo/s "porterage" (LSJ s.v.). "Porter" was commonly used as a term of abuse; see LSJ s.v. prou/neikos, fo/rtac, fortiko/s.
Chantraine calls the etymology "obscure."
[2] It is likelier for the noun ko/balon "knavery" to be derived from ko/balos "knave".
[3] From the scholia to Aristophanes, Knights 270 (cf. next note): "ko/baloi are robbers holding pieces of wood; they are also called club-bearers." (Properly, korunofo/ros refers to bodyguards: LSJ s.v.) The scholion is also cited in upsilon 393.
[4] Aristophanes, Knights 270; the misconstrual of the verb as "rob", instead of "trick, act as a rogue to", comes from the scholiast's folk etymology with "robber".
[5] Also from the scholiast, conceding the "trickery" interpretation of the stem.