1. pile
[noun]
[de sta-pel, de sta-pels]

A "stapel" is a pile of books, rocks, papers, problems, etcetera. The related verb is "stapelen" or "opstapelen" ("to pile up"). A little pile is a "stapeltje".

Examples:
– "Ondanks de ‘clean-desk policy’ ligt het bureau van mijn collega altijd vol met stapels papier." 
("Despite the clean-desk policy, my colleague’s desk is always filled with piles of papers.")

– "Toen hij terugkwam van vakantie lag er een stapel post op hem te wachten." 
("When he returned from vacation, a pile of mail was waiting for him.")

– "In haar kast liggen alle broeken, rokjes, truien en bloezen netjes opgevouwen op stapeltjes." 
("In her closet all pants, skirts, sweaters and blouses are nicely folded and piled up.")

– "Omdat zij al haar geld verloor aan gokken, stapelden haar schulden zich op." 
("Since she lost all her money to gambling, her debts piled up.")

Expressions:
– "Op stapel staan": to be planned.

Related words:
– (Op)stapelen: to pile up [regular weak verb] [stapelde op, opgestapeld].
– Hoop: 1. hope [noun] [de hoop, <no plural>]; 2. heap [noun] [de hoop, de hopen].
– Ophopen: to pile up [regular weak verb] [hoopte op, opgehoopt].
– Berg: 1. mountain [noun]; 2. heap [noun] [de berg, de bergen].

2. madly in love
[adverb/adjective]

The second meaning of "stapel" is "madly in love". It is derived from "stapelverliefd" which means "madly in love". Another adjective that is often precede by "stapel" is "stapelgek" which either means "totally crazy" or "crazy in love" or both 😉 Listen to the hitsingle "Stapel op jou" by one of the best-known Dutch singers Marco Borsato.