Wikipedia
Clinamen (; plural clinamina, derived from clīnāre, to incline) is the Latin name Lucretius gave to the unpredictable swerve of atoms, in order to defend the atomistic doctrine of Epicurus.
According to Lucretius, the unpredictable swerve occurs "at no fixed place or time":
When atoms move straight down through the void by their own weight, they deflect a bit in space at a quite uncertain time and in uncertain places, just enough that you could say that their motion has changed. But if they were not in the habit of swerving, they would all fall straight down through the depths of the void, like drops of rain, and no collision would occur, nor would any blow be produced among the atoms. In that case, nature would never have produced anything.
This swerving, according to Lucretius, provides the "free will which living things throughout the world have."
In English it implies that one is inclined or biased towards introducing a plausible but unprovable clinamen when a specific mechanism cannot be found to refute a credible argument against one's hypothesis or theory. Lucretius never gives the primary cause of the deflections. The OED gives its first recorded use in English by Jonathan Swift in his 1706 Tale of Tub ix.166 where he ridicules an unsubstantiated argument:
The Round and the Square, would by certain Clinamina, unite in the Notions of Atoms and Void.
Usage examples of "clinamen".
He has to admit, however, that atoms do not aggregate of their own accord, and rather than believe in a superior law and, finally, in the destiny he wishes to deny, he accepts the concept of a purely fortuitous mutation, the clinamen, in which the atoms meet and group themselves together.
Epicurus modestly hoped, that one time or other's certain fortuitous concourse of all men's opinions, after perpetual justlings, the sharp with the smooth, the light and the heavy, the round and the square, would by certain clinamina unite in the notions of atoms and void, as these did in the originals of all things.