Alaikumassalam wa rahmatullah,
Ismahaan is a name used in Arabic and Turkish. It is formed from Ism (name) in Arabic and Haan, which comes from Khan, which means "ruler", "sir", "lady". Asmaa is another popular name that comes from Ism. The full meaning of Ismahaan is not clear,
It is also possible that it is a shortened form of Ismat Hanum, which means "lady Ismat", a very famous Muslim woman who was wife of Saladin. It is possible that out of love and admiration for her, Muslims started to name their daughters Ismat Hanum and as time passed the name was shortened to Ismahaan.