This paper aims to analyse the Hittite form liliwaḫḫuwanzi ‘quickly, soon’,with a focus on its origin and function. It will be suggested that this infinitive form underwent a process of lexicalisation, whereby an inflected form was frozen into a new item, becoming a modal adverb. This implies the introduction of a new lexeme to the lexicon. Focusing on the finite forms of liliwaḫḫ-‘to make haste, hasten’, the paper will propose how such a class change occurred via lexicalisation.