glatt
... is German for => without a remainder
smooth means: having a fine surface which is not felt when stroking it.
without a remainder means: a division produces a natural number as a result.
Examples
German: Seide ist normalerweise sehr glatt.
English: Silk is usually very smooth.
German: 20 geteilt durch 5 geht glatt auf.
English: 20 divided by 5 has no remainder.