“know sb inside out” 起源于20世纪初，“inside out” 本指“内外翻转”，后引申为“从本质到表象的全面覆盖”，短语核心语义为“彻底了解某人”，侧重对他人性格、习惯、优缺点、思维方式等全方位掌握，强调了解的深度与完整性。know sb like the back of one's hand（最常用替换）如：
I’ve known her inside out since we were kids.
我们从小一起长大，我对她了如指掌。
After 20 years of marriage, he knows his wife inside out.
结婚20年，他彻底了解他的妻子。
She claims to know the candidate inside out and can vouch for his integrity.
她声称对这位候选人了如指掌，能为他的正直作担保。
We’ve been dating for a year, and I know him inside out—he’ll never lie to me.
我们约会一年了，我太了解他了，他绝不会骗我。
As a teacher, she knows each student inside out and tailors her teaching accordingly.
作为老师，她对每个学生都了如指掌，并据此调整教学。
The economist knows the global market inside out and predicted the recession accurately.
这位经济学家对全球市场了如指掌，准确预测了经济衰退。
They’ve been best friends for decades—she knows him inside out, even his deepest fears.
她们是几十年的挚友，她对他了如指掌，包括他最深的恐惧。
The diplomat knows the negotiating partner inside out and found a breakthrough.
这位外交官对谈判对手了如指掌，找到了突破口。
The general knows the enemy’s tactics inside out and made a precise plan.
这位将军对敌人的战术了如指掌，制定了精准的计划。
I’ve known Ross inside out for years—he’s just too proud to admit he’s wrong.
我认识罗斯这么多年，对他了如指掌——他太骄傲，不肯承认错误。《老友记》
We had been roommates for three years, and I thought I knew her . inside out. But when she suddenly decided to quit her job and travel around the world, I was totally shocked.（2023年高考英语全国甲卷）
