The pleasant route through life, of pleasure and dissipation.
This phrase was coined by Shakespeare, in Hamlet, 1602. It is evidently a simple allusion to a path strewn with flowers.

Ophelia:
I shall the effect of this good lesson keep,
As watchman to my heart. But, good my brother,
Do not, as some ungracious pastors do,
Show me the steep and thorny way to heaven;
Whiles, like a puff’d and reckless libertine,
Himself the primrose path of dalliance treads,
And recks not his own rede.