French dropped /s/ at the start of consonant clusters, at the start and in the middle of words. So /sp/ > /p/, /sn/ > /n/, /st/ > /t/ etc: hospital > hôpital, Rhodanus > *Rhodne > *Rhosne > Rhône, establir > établir.
The motivation for this in phonotactics is to reduce the complexity of syllable structures, which make syllables easier to pronounce. It is linguistically common for /s/ to drop off at the start of clusters like that. Dennis Rhodes’ answer notices a similar process going on in some dialects of Spanish (Cuba, Puerto Rico): español ends up pronounced as etpañol, which is on the way to epañol. In my favourite language, Tsakonian, /s/ followed by a consonant is replaced by the aspirated consonant: stoma > tʰuma, sporos > pʰore.