The meaning of 'kapdan'

In Avot 2::6 Hillel teaches that a kapdan cannot teach.  What exactly is a kapdan?  ArtScroll publications render it as "A quick, impatient person". Rabbi Jonathan Sacks' translation in the Authorised Daily Prayer Book is broadly in agreement, except that he limits the meaning to just "impatient" [question: is there actually such a thing as a slow, impatient person?].  Herbert Danby (The Mishnah) and Hyman E. Goldin (Ethics of the Fathers) agree.

There are however other renderings: Irving M. Bunim (Ethics from Sinai) favours "quick-tempered", while David N. Barocas (in his translation of Me'am Lo'ez) goes for "overparticular".  R. Travers Herford (Ethics of the Talmud) gives "passionate". Jastrow's Dictionary offers "hot-tempered" (as does the Soncino Talmud translation of kapdan in Sanhedrin p 113b), "impetuous", meanings which chime in well with his account of the related verb kefad -- for which he also offers the meanings of "to care for", "to mind".

Our Sages also display a variety of views. For example the commentary ascribed to Rashi describes a kapdan as a teacher who is katzer lev (irritable?) and who gets angry with talmidim who ask him questions many times on the same topic. The Bartenura pictures him as someone who plainly doesn't like receiving questions from his talmidim and who doesn't give clear answers. 

The same three-letter root of קפד is also present in the word kipod -- a hedgehog.  Somehow the image of this small mammal works better than any verbal formulation in expressing the qualities of a kapdan: he is a prickly individual and one has to be careful how one approaches him. From the point of view of the rabbi-talmid relationship there are serious problems if the talmid is inhibited from approaching his teacher -- which is why someone with the characteristics of a kapdan may not be best placed to teach.