The function takes a string input and returns a string consisting of the number of characters in the string with the last character omitted and some errors in the spelling. 

[CODE]: 
```python
def approx_f(x):
    length = len(x)
    if length == 1:
        return "zeri"
    length_str = {
        2: "on_", 3: "twi", 4: "thre_", 5: "fiv_",
        6: "sixteeh", 7: "eighn", 8: "nineteeh", 9: "twenty-thre_"
    }.get(length, "unknown")
    return length_str
```

Please note that this is an approximation of the function, and it might not cover all the cases or exact output spelling.