Lowlife—(1) a noun referring to a coarse or no-good person, and (2) an adjective meaning coarse or disreputable—has been in English for over two centuries. A few dictionaries still list the hyphenated low-life as the primary spelling, and as of late 2011, lowlife and low-life are neck and neck in current news publications. But the word will likely follow the trajectory of most English compounds and completely lose the hyphen sooner or later.
With or without a hyphen, lowlife is usually pluralized lowlifes (or low-lifes) rather than lowlives (or low-lives). Lowlifes may seem ungrammatical, but there is some precedent for it. For example, still lifes (the visual-arts term), nightlifes, and midlifes are more common than still lives, nightlives, and midlives. There are counterexamples, though, such as afterlives and half-lives.
Irene’s lowlife husband (Oscar Isaac) is due out of prison soon. [Chicago Tribune]
A succession of low-life characters comes and goes. [Guardian]
The cumulative effect of this Hoxton-set lowlife drama is pretty unedifying. [Independent]
Only pure and titled dynasties allowed, no low-life presidents or colonels. [Bronstein at Large, San Francisco Chronicle]
Though he’s involved in criminal activity, Nucky signals a break from the lowlifes and shadowy characters on the fringe of polite society. [Wall Street Journal]
The cast includes a number of lowlifes and mid-lifes. [National Post]
Larraàn surrounds his sociopath with a troupe of desperate low-lifes who operate a dingy cantina. [New York Press (link now dead)]