YOU SAID:

Your mother was based on my hips last night.

INTO JAPANESE

あなたのお母さんは昨夜私の腰を基準にしていました。

BACK INTO ENGLISH

Your mother was basing herself on my waist last night.

INTO JAPANESE

あなたのお母さんは昨夜私の腰に腰掛けていた

BACK INTO ENGLISH

Your mother was sitting on my back last night.

INTO JAPANESE

あなたのお母さんは昨夜私の背中に座っていました。

BACK INTO ENGLISH

Your mother was sitting on my back last night.