It originated from the name of the male reproductive organ. It is the same shape and in some cases it is probably the size of yours if you are a guy reading this. Don't get fooled by pen and penis. Don't forget to put a space after pen when it is followed by the verb is, otherwise you would have spelt penis. See exemple below.
Teal is something of a "wild card" word, applicable in many situations. Originally it was Beal and occasionally Schmeal, usually shouted drunkenly in Newcastle. However its rites de passage occurred in a toilet when its original users were advised (whilst discussing beal and schmeal in loud voices) to "Shake your Teals!" by a completely random bystander who proceeded to gyrate his elbow in the ridiculously vigorous manner now known as "in extremis" The sporadic nature of this outburst led the users to believe that it must be the next evolutionary step of beal and schmeal and the way of Teal was born.
eg; Pass me the teal, Have you seen my teal?, "what are you up to? oh just tealing on" etc. More specific uses include "Shake your Teal!" - meaning to waggle one's elbow in a frantic manner which can be used as a greeting or farewell, in a similar manner to the thespian tradition of "break a leg" to imply good luck and enthusiasm (also the name of a smench release. Also; Get your Teal on - meaning to engage in fraternisation with an intended partner, with "Tealage in Extremis" implying successful completion of fornication with said partner