"The piercing star." The star (al-najm) in this context refers to the collection of stars, and the 'piercing' (al-thaqib) means the luminous/shining.
Muhammad said: It is said: thaqaba, yathqubu, thuquban, when it shines/illuminates. It is also said to one who kindles a fire: 'Athqib naraka' (Kindle your fire), meaning: light it. This is an oath.
﴿النَّجْم الثاقب﴾ وَالنَّجْمُ فِي هَذَا الْمَوْضِعِ جَمَاعَةُ النُّجُومِ، وَالثَّاقِبُ: الْمُضِيءُ.
قَالَ مُحَمَّدٌ: يُقَالُ: ثَقَبَ يَثْقُبُ ثُقُوبًا إِذَا أَضَاءَ، وَيُقَالُ لِلْمُوقِدِ: أَثْقِبْ نَارَكَ، أَي: أضئها. وَهَذَا قسم.
