変てこ
is a shortened form of 変てこりん
, which seems to have underwent phonological change from 変ちくりん
. Not sure about the motivation of the sound change, but does not look unreasonable. The vowels in ちく
changed:
[iu] → [eo], which is:
[+high, -back][+high, +back] → [-high, -back][-high, +back]
The origin of ちくりん
is unknown, but is used with a few na-adjectives such as
変ちくりん
珍ちくりん
妙ちくりん
But be careful because some of them are politically incorrect.
ちくりん
can be considered as a diminutive, adding cuteness. 変な
does not have that cuteness nuance. 変てこ
, 変てこりん
, and 変ちくりん
imply funny thing that is human made and cannot be used in a serious situation, whereas 変な
simply means strange or unusual.
Scene: A doctor is doing a surgery, and finds an unusual thing in the patients body.
In this case, the doctor can say 変なものがある
, but definetely not 変てこなものがある
.
Scene: There is a painting on the wall, which is commical and fun but nothing wrong.
In this case, although 変な絵がある
is not impossible, 変てこな絵がある
will be a better fit.
As Tsuyoshi Ito mentions in the comment, へなちょこ
means poor in quality. It does not mean strange.