If you toss that phrase into a fanfic search today you'll see it everywhere, and honestly that's how I kept bumping into 'undulating kiss' the most — in passionate, breathy scenes where someone wants to make a kiss feel almost physical in motion. The word choice is clever: 'undulating' makes the scene move, not just happen, and in fandom we love verbs and adjectives that make hearts thump or make a whole moment cinematic.
From a reader's point of view, the phrase probably didn't spring from one
famous novel so much as from a habit writers have of borrowing natural imagery for intimacy — think waves, tides, throbbing hearts. You can trace that impulse back to Romantic poets who used nature as metaphor in works like 'Ode to a Nightingale' or to the theatrical language of earlier plays like '
romeo and juliet' where passion is often likened to storms and tides. In contemporary practice, it gained traction in cheap romance paperbacks and online stories where vivid, sensory language rules. So when I see it now, I immediately picture a slow, cinematic kiss — not just a peck, but a sequence with rhythm and a little gasp afterward. It reads modern but rooted in older metaphor traditions, and I kind of enjoy how theatrical it sounds.