The function takes a string input and returns the length of the input string as an English word (e.g., "five" for an input of length 5).

[CODE]: To approximate the behavior of the function in Python, you can use the following code:

```python
def string_length_in_words(input_str):
    num_to_word = {
        1: "one", 2: "two", 3: "three", 4: "four", 5: "five",
        6: "six", 7: "seven", 8: "eight", 9: "nine", 10: "ten",
        11: "eleven", 12: "twelve", 13: "thirteen", 14: "fourteen", 15: "fifteen",
        16: "sixteen", 17: "seventeen", 18: "eighteen", 19: "nineteen", 20: "twenty"
    }
    input_length = len(input_str)
    return num_to_word.get(input_length, "unknown")
```

This code approximates the behavior of the function by mapping the length of the input string to its corresponding English word. If the input length is not within the dictionary's keys, it returns "unknown".