Eel : noun A eels is a fish very elongated in shape, with viscous and slippery skin, which grows in fresh water and reproduces at sea, whose flesh is widely consumed, particularly in northern countries.
Slide like an eel: escape.
Playing the eel: being elusive, not recognizable.
Boxer eel: penis.
Have a nice eel: erect black sex.
There is eel under rock : There is something hidden, a perfidy that we suspect, which is brewing; the matter is not clear.
The eel is a fish scavenger that lives mainly at night; during the day, it therefore has a tendency to hide and preferably under the rocks. Our expression therefore corresponds to a reality. But it is also a metaphor that can be explained in other ways.
According to Pierre Guiraud (1912-1983), linguist of the twentieth century, in French phrases, the meaning of "hidden deception" would come from the link established more or less consciously or from a play on words between eels and two verbs guiller from old French.
The first had the meaning of "avoiding the fight, sneaking", a bit like the eel trying to escape.
The second guiller comes from francic wigila ("Trick, trick") and meant "to deceive" hence also the denomination of Guillaume to suggest deception.
Finally, the eel was often likened to a snake, a deceitful animal if there was one. This expression is attested in 1532 by the writer François Rabelais (1494-1553), but it is probably older.