The term "sneaky fuckers" was coined by evolutionary biologist John Maynard Smith to describe subordinate males who take advantage of the opportunity to mate with females while dominant males are otherwise occupied, leading to their reproductive success (Smith, 1993). It was originally thought that in some species such as deer and gorillas, only the dominate male mates successfully. But, through direct observation and DNA analysis, it is now known that often other males surreptitiously successfully mate when they can find the opportunity.
This successful mating strategy was also documented in the Giant cuttlefish. While large males compete in physical combat to win mates, smaller males will often adopt the coloration patterns of females to "sneak" by competing males and successfully mate with females. This is shown beautifully in the NOVA documentary, Kings of Camouflage at minute 24 http://www.pbs.org/wgbh/nova/nature/kings-of-camouflage.html.
A twist to natural selection theory
In Darwin's seminal work Origin of a Species, he lays the foundation of the sexual selection theory, whereby some individuals are able to have higher reproductive success as a result of being able to outcompete for mates. The "sneaky fucker theory" flies in the face of the precepts of the sexual selection theory, and sets up a scenario where successful mating is not necessarily limited by the ability to attract a mate. John Maynard Smith cited the example of the red deer where subordinate males will take advantage of a mating opportunity while the dominant males are engaged in combat.
The "sneaky fucker theory" has been discussed in connection with why homosexuality has not been selected out by natural selection and is discussed in this video interview with Richard Dawkins:
Photo of John Maynard Smith credited to : npg.org.uk
Darwin, C. (1859). On the origin of the species by natural selection.
Smith, J. M. (1993). Evolution and the Theory of Games (pp. 202-215). Springer US.