adjective
secret, secretly
B1
Heimlich means secret (adj) or secretly (adv). It’s gradable (comparative heimlicher; superlative am heimlichsten) and contrasts with offensichtlich/offen. As an adjective it describes concealed things or intentions; as an adverb it modifies verbs (e.g., he looked away heimlich). Not reflexive or modal; regular adjective.
Examples
Sie hatte eine heimliche Überraschung für ihn.
She had a secret surprise planned for him.
Er hat heimlich das Geschenk geöffnet.
He secretly opened the present.
Die Firma wurde kritisiert, weil sie heimlich Kundendaten sammelte, obwohl dies gesetzlich verboten war.
The company was criticised because it secretly collected customer data, although this was legally prohibited.
Details
Mnemonics
Imagine someone slipping a note under a floorboard by moonlight — a secret act.
Sounds like 'hide-ly' — think of hiding to remember 'secret'.
Notes
Used both as an adjective (eine heimliche Sache) and as an adverb (etwas heimlich tun). Remember context to choose translation.