From Halopedia, the Halo wiki
Señora Sies was a modified M6-series handgun used by Ignatio Delgado. According to Delgado, the original furniture had been replaced with oak wood, which was very scarce on Madrigal, and had been carved into "a piece of art" over the course of three weeks by his uncle.
As ODST Major Faison lay dying aboard the Kig-Yar raider Infinite Spoils, Delgado exchanged Señora Sies for the major's battle rifle. The handgun was presumably lost after Faison was killed by the Sangheili Zealot Zhar, and Thel 'Vadamee kicked away the weapon.
The handgun's name is likely a misspelling of "Señora Seis", Spanish for "Mrs. Six", a play on the weapon being an M6 sidearm.
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