The function takes an input string and returns the word form of the length of the input string.
[CODE]: 
```python
def f(x):
    word_lengths = {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"}

    length = len(x)

    if length > 20:
        return "twenty"
    
    return word_lengths[length]
```
This approximated code might not cover all the cases, but it should be very close to the actual function's behavior.