A passiuncle is an insignificant or trivial passion. The word was invented by De Quincey to describe those emotions whose exercise exhausts the soul and the heart, making them incapable of true passion.
Such a failing is evident in Romeo who, you will recall, begins the play madly in love with a girl called Rosaline. As Friar Laurence says:
Holy Saint Francis, what a change is here!
Is Rosaline, whom thou didst love so dear,
So soon forsaken? young men's love then lies
Not truly in their hearts, but in their eyes [...]
And art thou changed? pronounce this sentence then,
Women may fall, when there's no strength in men.
Although Auden may have put it better in his sequel to The Tempest:
Will Ferdinand be as fond of a Miranda
Familiar as a stocking? Will Miranda who is
No longer a silly lovesick little goose,
When Ferdinand and his brave world are her profession,
Go into raptures over existing at all?
And while we're on the subject, too few people are aware that E.M. Forster wrote a brief sequel to A Room With A View (called A View Without A Room), in which George Emerson, forgets all his peacenik principles and cheats prolifically on Lucy Honeychurch.
For myself, I am certain that I am incapable of true emotion, but I inhabit a delightful aviary filled with flittering passiuncles and squawking whimsies.