Pronunciation: [fˈɔːl θɹˈuː] (IPA) 
The correct spelling of "fall thru" is actually "fall through". The word "through" is pronounced with a long "u" sound like "threw" /θru/. It may appear that the word should be spelled "thru" but this spelling is considered informal and not standard in English. When writing formally, it is important to use the standard spelling "through". So, remember to fall through an open door and not fall thru it!
The term "fall thru" is an informal expression derived from the more widely recognized phrase "fall through." With a very similar meaning, "fall thru" is primarily used in casual settings, particularly in spoken or written conversations among friends, colleagues, or informal online communication.
"Fall thru" essentially refers to a situation where a plan, arrangement, agreement, or commitment fails to materialize as intended, thereby leading to its cancellation or non-fulfillment. It implies that an anticipated event, action, or arrangement did not come to fruition for various reasons, thereby resulting in disappointment, frustration, or inconvenience.
This term is commonly used to describe circumstances where someone or something did not follow through with their part of an agreement or a commitment, leading to the failure or cancellation of an intended action. For instance, if a friend had promised to meet you for dinner but cancels at the last minute without a valid reason, you could say, "Our dinner plans fell thru."
In essence, "fall thru" refers to a situation where plans, promises, or agreements fail to materialize or are not honored, often leaving individuals or parties in a situation different from what was expected. This term is typically used informally to acknowledge and express disappointment or frustration when things do not go as planned.
The term "fall through" is a phrasal verb that originated from the combination of the word "fall" and the word "through".
The word "fall" has Old English and Germanic origins, coming from the West Germanic word "fallan" or "faldan", which means "to fall". It has been used in various senses throughout history, denoting dropping or descending from a higher position to a lower one.
The word "through" also has Old English roots and can be traced back to the Proto-Germanic word "thoruh", meaning "through" or "across". It refers to moving in one side and out the other, passing from one place to another, or getting to the conclusion or completion of something.
When combined, "fall through" forms a phrasal verb, which means to fail or not meet expectations or to go wrong.
