Theory:

     On another occasion, I knocked a soft drink onto the lap of a sweet little lady sitting beside me. The flight attendant came and cleaned her up, and brought me a replacement drink, and instantly I knocked it onto the woman again. To this day, I don’t know how I did it. I just remember reaching out for the new drink and watching helplessly as my arm, like some cheap prop in one of those 1950s horror movies with a name like The Undead Limb, violently swept the drink from its perch and onto her lap.
 
     The lady looked at me with the stupefied expression you would expect to receive from someone whom you have repeatedly drenched, and uttered an oath that started with “Oh”, finished with “sake” and in between had some words that I have never heard uttered in public before, certainly not by a nun.
Explanation:
 
Later, the narrator recounts yet another incident that happened while he was on a plane. He accidentally knocked a soft drink onto the lap of his fellow passenger. It is interesting to note that the narrator describes the passenger as "a sweet little lady". This description is essential because it adds more meaning and depth to the situation.

For instance, you could say that the narrator was feeling sorry for what he had done to the lady and, as a result, described her as sweet and little. Moreover, the narrator found her sweet because she never retaliated but remained calm. Or it could merely be a description of her physical stature. She was described as little because she was probably short. Regardless of the possible intention behind the adjectives, what follows next intensifies the mood.

After knocking the drink, the flight attender came, cleaned the lady up, and brought the narrator a replacement drink. As luck would have it, the narrator knocked it on the woman again!
 
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The narrator drops the drink (twice) on the lady
 
The lady was dumbstruck! She couldn't believe that the narrator would do it again. She was so astonished and taken aback that she muttered the unspeakable expression, "Oh for **** sake". The narrator was more than surprised to hear the lady utter the phrase in public, and more so because she was a nun.
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The lady mutters an oath
 
Thinking back to the incident, the narrator says that he could never figure out how such things happened. He felt like his arm acted on its own. He remembers reaching out for the replaced drink, and he watched helplessly how his clumsy arm moved abruptly and violently to knock the glass off. The narrator also compares his clumsy arm to a cheap prop from one of those \(1950s\) horror movies with a name like "The Undead Limb".
 
Meanings of difficult words from the paragraphs:
 
S.No
Words
Meanings
1
PerchA bar or makeshift table
2
StupefiedMake someone unable to think or feel properly
3
DrenchedWet thoroughly
4
Oath
Here, an offensive word or phrase
Reference:
National Council of Educational Research and Training (2006). Moments. Accidental Tourist- Bill Bryson (pp. 56-59). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.