meaning - Are there any rules when combining "semi-" with other words?

The definition of semi is half, partial. Examples like semi-annual/semiannual, semi-truck come up pretty often. However, I've gotten into a debate with some who used the term semi-sales to refer to a sales job that is not quite actually sales. My reasoning was that you can't simply combine semi- with just about anything and create new words or new meanings to a word.

Are there any rules to using semi-?

Answer

Michael Quinion, Ologies and Isms: Word Beginnings and Endings (Oxford, 2002) has this to say about the prefix semi-:

semi- Half, a part. {Latin semi-, half.}

Th strict sense of a half occurs only in a minority of words, of which examples are semicircle; semidiameter; semilunar; shaped like a half-moon or crescent; and semiquaver, in British musical terminology a note having the time value of half a quaver, a sixteenth note. A few terms extend the idea to that of occurring twice in some time period, as in semi-annual, occurring twice a year (nominally every half year), and the North American semi-monthly, occurring or publishing twice a month (or every half month). A related idea occurs in semi-final, a match or round immediately preceding the final, the 'half-final'.

Most terms in the prefix, however, signal that something is partially or incompletely so: semi-professional, semi-conscious, semi-retired, semi-literate, semi-skilled, semi-derelict, semiprecious, semiconducting (of a substance that has a conductivity between that of an insulator and that of most metals), and semipermeable (of a material or membrane that allows certain substances to pass through it but not others).

A similar analysis could be applied to English words that begin with half-: there are the exact one-half words (half-crown, half-dollar, halfmoon, half-hour), the approximately one-half words (half-cell, half-life, half-mast, halftone), and the the words where half- really just means partial (half-baked, half-cocked, halfhearted, half-light, half-truth).

Under the circumstances, I have trouble drawing a bright line between semi-skilled in "a semi-skilled position" and semi-sales in "a semi-sales position" and saying that the former constitutes a legitimate use of semi- but the latter does not.