What does -komu (ー込む) at the end of a word mean?

Question

There are loads of word in Japanese which end in 込む, like 吸い込む, 読み込む, 入り込む, 打ち込む, 売り込む, 送り込む, 押し込む. How does adding ー込む change the meaning? What is the meaning that links all these words?

Answer 1

込む by itself can be interpreted as a intransitive verb to mean "jammed in", "packed in", "crowded". For example 電車が込む

However, when 込む is used with compound verbs it can have 3 different interpretations.

1. ‘to enter; put something in/into’

   Implies a physical transition where an object (or a person) shifts from a place into an enclosed location.

   Examples
   雨が吹き込む - Rain blows in.
   飛び込む - Jump in/Dive.

2. 'to do/become something thoroughly/fully/deeply/intensively'

   Normally denotes that someone undergoes a certain physical/psychological occurrence that produces physical/mental changes or development.

   Examples
   教え込む - Inculcate a thing (in a person’s mind); instill into (a person); give a good training.
   思い込む - Be convinced; be under the impression; set one’s heart; fall in love.

3. ‘to continue an action within the same condition’

   Indicates that someone is involved in a physical recurrent action.

   Examples
   黙り込む - Fall silent; keep one’s mouth shut; clam up.

Reference:
library.nakanishi.ac.jp/kiyou/gaidai(36)/09.pdf

Answer 2

Usually it means to do something more thoroughly, completely, or intensely.

Sometimes it can also be like adding "into" after the verb: 押す, "to push" becomes 押し込む, "to push into."

Answer 3

The examples given by the OP are pretty clear-cut, and covered well by Amanda's answer. Where 〜込む can get difficult is in the verbs that don't directly relate to an action.

For example, 思い込む, per Amanda's explanation, would mean to "think thoroughly, completely, intensely, or to be packed in"... not exactly intuitive for the exact meaning of "to wrongly convince oneself of/talk oneself into something".